Preheat the oven to 350°F (180°C).
Sift the flour into a mixing bowl and then add the sugar, baking soda, salt and cocoa powder and mix.
Prepare your vegan buttermilk by adding 1 tablespoon of lemon juice to a measuring jug and then adding soy milk up to the 1 cup line. Allow to sit for a minute to curdle.
Add the vegan buttermilk, vanilla, oil, vinegar and blue food coloring to the mixing bowl and mix into a batter. Don't overmix.
Line a cupcake tray with cupcake liners and divide the batter evenly between them.
Place into the oven to bake for 20-25 minutes or until a toothpick inserted into the center of one of the cupcakes comes out clean.
Transfer the cupcakes to a cooling rack and allow to cool completely before frosting.
Add all frosting ingredients except soy milk to your mixing bowl and whisk with an electric whisk starting on slow and gradually increasing speed until thick, smooth and creamy. Add soy milk as needed so that you get a consistency that can be easily piped or spread.
If you want to use some sanding sugar to decorate, add a couple of drops of liquid blue food coloring to about ¼ cup of white sugar in a jar. Close the lid of the jar and shake the jar vigorously so that the blue dye distributes evenly resulting in perfectly even blue sanding sugar.
Sprinkle on top of the frosted cupcakes.